This invention relates to RF (radiofrequency) devices and methods for delivering RF energy to tissue in a patient""s body, and more particularly to an electrode array that allows for controlled low-power RF energy delivery in orthopedic applications, for example in capsular shift procedures.
Joint instability in adults is caused by ligaments and cartridge in a joint becoming lax or stretched, due either to the aging process or to acute trauma. Joint instability is a widespread disease and is estimated to affect up to 10 percent of the male population in the U.S. A patient""s shoulder joints, knees, ankles and elbows all may become unstable due to lax ligaments. As a specific example, a patient""s shoulder joint (or glenohumeral joint capsule) is maintained in a stable condition by a capsular ligament complex, subscapular tendons, rotator cuff and teres minor muscles, among others.
Joint instability is caused by laxity in the fibrous ligament complex within the joint capsule. An increase in ligament laxity may be due to an acute-event type of trauma or recurrent minor trauma (i.e., wear-and-tear). Often, acute-event trauma results in a unidirectional type of instability, whereas normal wear-and-tear results in multidirectional joint instability. In terms of pathology, unidirectional joint instability may be defined as an excess capsular volume (space between the humeral head and synovial surface of the capsule) in a particular location, region or path across the capsule. Multi-directional joint instability generally may be considered to be excessive volume within the entire joint capsule around the humeral head.
Surgeons have developed open surgical treatments for reducing the volume of unstable joint capsules, generally termed xe2x80x9ccapsular shift proceduresxe2x80x9d. In such surgery, over-stretched or lax capsular ligaments are tightened and secured around the perimeter of the joint capsule. Such procedures frequently result in post-operative pain, loss of motion, nerve injury and even osteoarthritis. Further, capsular shift patients require lengthy post-operative rehabilitation and often do not achieve pre-injury levels of joint stability.
Surgeons also have developed minimally invasive arthroscopic techniques for performing capsular shift procedures which, for the most part, replicate the open procedures. An arthroscopic approach typically results in less post-operative pain and reduced rehabilitation time. However, arthroscopic capsular shift techniques require high levels of technical expertise. Also, it is not clear whether arthroscopic ligament fixation devices and methods are equal to those available in an open surgical approach.
More recently, to avoid surgical reconstruction of a joint capsule, arthroscopic surgeons have investigated the use of thermal energy to tighten or shrink the ligaments within a joint capsule. A capsular ligament complex includes various types of collagen, which is one of the most abundant proteins in the human body. It is well-known that collagen fibrils will shrink in length when subjected to temperatures ranging above about 60xc2x0 C. Interstitial collagen consists of a continuous helical molecule made up of three polypeptide coil chains. Each of the three chains is approximately equal in longitudinal dimension with the molecule, being about 1.4 nm in diameter and 300 nm in length along its longitudinal axis in the helical domain portion.
Collagen molecules polymerize into chains in a head-to-tail arrangement generally with each adjacent chain overlapping another by about one-fourth the length of the helical domain. The spatial arrangement of the three peptide chains is unique to collagen, with each chain existing as a right-handed helical coil. The superstructure of the molecule is represented by the three chains that are twisted into a left-handed superhelix. The helical structure of each collagen molecule is bonded together by heat labile cross-links between the three peptide chains providing the molecule with unique physical properties, including high tensile strength and limited longitudinal elasticity.
The heat labile cross-links may be broken by thermal effects, thus causing the helical structure of the molecule to be destroyed (or denatured) with the peptide chains separating into individually randomly coiled structures of significantly lesser length. The thermal cleaving of such cross-links may result in contraction or shrinkage of the collagen molecule along its longitudinal axis by as much as one-third of its original dimension. It is such thermal shrinkage of collagenous ligament tissue that can stabilize a joint capsule.
Collagen shrinks within a specific temperature range, (e.g., 60xc2x0 C. to 70xc2x0 C. depending on its type), which range has been variously defined as: the temperature at which a helical structure collagen molecule is denatured; the temperature at which xc2xd of the helical superstructure is lost; or the temperature at which the collagen shrinkage is greatest. In fact, the concept of a single collagen shrinkage temperature is less than meaningful, because shrinkage or denaturation of collagen depends not only on an actual peak temperature but on a temperature increase profile (increase in temperature at a particular rate and maintenance at a particular temperature over a period of time).
Thus, collagen shrinkage can be attained through high-energy exposure (energy density) for a very short period of time to attain xe2x80x9cinstantaneousxe2x80x9d collagen shrinkagexe2x80x94the method used by all previously known devices (both laser and high-energy RF waves) for joint capsule shrinkage. These previously known treatments shrink collagenous tissue in a matter of seconds (e.g., 1-2 seconds).
Previously known methods of xe2x80x9cinstantaneousxe2x80x9d capsular collagen shrinkage with a high energy (40 to 60 watts) mono-polar RF probe (or similar high-energy laser) suffer from several significant drawbacks. In such an RF treatment (or laser treatment), the surgeon xe2x80x9cpaintsxe2x80x9d the tip of the RF probe across a section of a joint capsule targeted for collagen shrinkage. Because the collagen targeted for shrinkage generally lies well under the capsular surface, high RF energy levels are needed to cause shrinkage, typically 40 to 60 watts. These power levels, however, pose a substantial risk of ablating or perforating the synovial surface, which is highly undesirable.
Also, as depicted in FIG. 1A, it is difficult to xe2x80x9cpaintxe2x80x9d the RF probe tip (even though only 3-5 mm in diameter) across the targeted portion of the joint capsule due to the limited working space between humeral head H and capsule C, while still maintaining an adequate endoscopic view of the damaged or lax tissue indicated at D in FIG. 1A. At times, it may be necessary to use an lever-type instrument to pry (or retract) the humeral head away from the joint capsule to provide a larger working space, thus posing a risk of damaging the labrum (the fibrous cartilage surrounding glenoid capsule G).
Further, the previously known methods of creating xe2x80x9cinstantaneousxe2x80x9d collagen shrinkage cause the working space between the humeral head and capsular surface to shrink and disappear practically instantaneously, thus making it necessary to work from a first position treatment location L1 toward a second location L2. Thus, it is generally not possible to return toward the first location L1 for additional treatment or diagnosis (see FIG. 1A).
Previously known methods of xe2x80x9cpaintingxe2x80x9d tissue with high-energy RF waves with a hand-held probe to achieve rapid collagen shrinkage are not well suited for collagenous tissues of different thicknesses and/or for tissue in which collagen content varies. For example, the capsular regions carrying the medial and inferior glenohumeral ligaments have significant collagen content (e.g.,  greater than 85%) and are quite thick. Areas between the ligaments and around the axillary recess are quite thin. Other areas of the joint capsule contain much less collagen (e.g.,  less than 40%).
Thus, xe2x80x9cpaintingxe2x80x9d the synovial surface with RF wavesxe2x80x94even if the probe is moved at a steady ratexe2x80x94will not cause uniform capsular shrinkage. Such free-hand techniques are technically demanding with a steep learning curve. In practice, an experienced surgeon will xe2x80x9cpaintxe2x80x9d the RF probe tip across the capsular surface in high collagen areas, but will stop and hold the probe tip in firm contact with thicker ligament areas (or areas with lesser collagen), in order to apply sufficient heat to the tissue. Such start-and-stop motions, however, tend to pose a risk of ablating and perforating the synovial lining.
Moreover, there are disadvantages in using a hand-held mono-polar RF probe when relying on a thermal sensor in the probe tip to safeguard against surface tissue ablation. While thermal sensors are often touted as having the ability to cut off RF delivery when tissue exceeds a certain temperature, this is generally the case only when a tissue mass is firmly in contact with the sensor. In the above-described xe2x80x9cpaintingxe2x80x9d techniques, however, the probe tip contacts the tissue with varying pressures, so that the xe2x80x9cactualxe2x80x9d tissue temperature may vary greatly from the temperature detected by the probe. Again, there is a substantial risk that the synovial surface may be ablated or perforated by excessively high temperatures before RF current flow is terminated.
Still other disadvantages of the previously known apparatus and methods are associated with high-energy mono-polar RF delivery. RF energy causes thermal effects in a tissue mass by perturbation or agitation of ions as alternating RF energy courses through the tissue in random paths of least resistance between the active mono-polar RF electrode and a ground plate. As depicted in FIG. 1B, xe2x80x9cpaintingxe2x80x9d a mono-polar RF probe tip across a synovial surface causes the RF paths through tissue (to the ground plate) to change constantly, preventing the perturbation of ions in any particular path or location and thus preventing effective energy densities from being attained in any particular location.
Previously known methods thus achieve xe2x80x9cinstantaneousxe2x80x9d collagen shrinkage only by using a very high current intensity (for high energy densities) that are compatible with the moving electrode (xe2x80x9cpaintingxe2x80x9d) technique. As shown in FIG. 1B, the RF current paths are only momentarily in a given position and not focused on the tissue that is targeted for ionic agitation. Ideally, as shown in FIG. 1C, the portion of capsular ligaments (depthwise) that need to be heated is indicated by the shaded area.
Yet another disadvantage of previously known mono-polar RF probes relates to the focus of RF energy created around the probe tip. The small diameter of the probe tip (e.g., from 2 mm to 5 mm for reaching into the joint capsule) when energized at high power levels causes the a focus of RF energy at the probe tip. Again, such small diameter mono-polar RF electrodes require much higher energy levels than would be required of a larger electrode to achieve a given level of thermal effects in the joint capsule.
In view of the foregoing, it would be desirable to provide apparatus and methods for elevating the temperature of collagen tissue in a joint capsule that preferably (i) utilize relatively low RF power levels to prevent surface ablation, (ii) are adaptable for treating tissues having high and low collagen content, and (iii) allow for observation of the shrinkage at less than an instantaneous rate.
It also would be desirable to provide apparatus and methods that shrink collagen at lower rates and at a lower temperatures than obtained with previously known RF apparatus and methods.
It further would be desirable to provide apparatus and methods that create a uniform or predictable path for RF current flow through targeted tissue, thereby causing more uniform heating of tissue to a low-rate collagen shrinkage temperature.
In view of the foregoing, it is an object of this invention to provide apparatus and methods for elevating the temperature of collagen tissue in a joint capsule that preferably (i) utilize relatively low RF power levels to prevent surface ablation, (ii) are adaptable for treating tissues having high and low collagen content, and (iii) allow for observation of the shrinkage at less than an instantaneous rate.
It also an object of the present invention to provide apparatus and methods that shrink collagen at lower rates and at a lower temperatures than obtained with previously known RF apparatus and methods.
It is a further object of this invention to provide apparatus and methods that create a uniform or predictable path for RF current flow through targeted tissue, thereby causing more uniform heating of tissue to a low-rate collagen shrinkage temperature.
These and other objects of the present invention are accomplished by providing an electrode array comprising an elongated insulator strip having at least one pair of spaced-apart bi-polar RF electrodes, and a xe2x80x9cchannelingxe2x80x9d electrode disposed on the strip between the bi-polar electrodes to direct the flow of RF current therebetween. The channeling electrode is not directly coupled to the RF power source, but coupled only indirectly through the tissue in contact with the channeling electrode. The apparatus enables low RF power levels (e.g., 0.5 watts to 25 watts) to be used to attain low-rate collagen shrinkage by directing or focusing the path of the RF current.
In a preferred embodiment, bi-polar electrodes are provided in first and second groups at each end of an elongated insulator strip adapted to be inserted into a joint capsule through a cannula. The bi-polar electrodes are exposed on one surface of the strip, and are connected to a suitable RF source by individual current-carrying wires. Any pair of bi-polar electrodes of the first and second groups may be selected to deliver RF energy. A channeling electrode is disposed on a central portion of the insulator strip, spaced apart from the bi-polar electrodes, with one surface exposed in the same direction as the active electrodes. The channeling electrode has no direct electrical connection to the RF source or any of the active electrodes.
Methods of using the apparatus of the present invention to perform capsular shift procedures are also provided.